Residents who have suffered the worst of June's extraordinary rains and floods are on alert for potential looting in areas where homes have been left empty and cars abandoned.

South Yorkshire police said extra-high-visibility patrols had been brought in because of residents' concerns and warned they would bring 'the full might of the law' down on anyone caught looting.

Bob Dyson, the force's deputy chief constable, said: 'There have been rumours of burglary and theft in empty homes but we have not received any reports of this. No one deserves to be a victim of crime but to take advantage of people affected by the flooding would be viewed severely by us. Our officers would bring the full might of the law to bear on anyone caught doing that.'

In Toll Bar, near Doncaster, resident Mark Birkby said four to six men from the village were keeping watch 24 hours a day because police were not doing enough. Residents said they had caught people trying to break into a car and stopped outsiders from entering the flooded area. 'We had nothing here at all; no help from police or council, nothing at all for days,' Birkby added.

Mary Dhonau, co-ordinator of the National Flood Forum (NFF), said she had reports of extra police patrols in Sheffield and a woman in Cumbria who had been burgled. Looting was also a problem after previous floods in Worcester, added Dhonau, who lives in the city. 'It's a widespread problem,' she said. 'That's the kind of stories people have to go through: every single person who's been flooded has ongoing issues [and] very often flooding can just break them.'

As emergency services were braced for fresh problems after bad weather brought more rain to already sodden areas, the Environment Agency said it was also preparing for the big clean-up. Peter Holmes, the agency's flood risk manager for the badly hit Ridings area in the north-east, warned people to stay out of the water to avoid the risk from rats and sewage that has been flooding out of drains, and to use protective clothing, gloves and disinfectant when they clean up their homes.

Holmes also warned homeowners to watch out for cowboy builders, who targeted people after the 2000 floods, carrying out shoddy work or taking money for promised work they didn't do. The agency advises residents to contact their insurance company.

Dhonau said: 'It's an absolutely terrible issue. I can almost hear the spurs heading north at the moment - the cowboys going up there. Even if it means you're going to be displaced for longer, wait until you can get a good builder.'

Tens of thousands of homes and businesses are expected to wait a week or more for water levels to recede, even without taking into account predictions of more violent storms this week. Experts warned that it would take months to repair the damage, at a cost now expected to top Britain's record of £1.3bn during the massive floods in the autumn of 2000. It is believed that seven people died as a result of the floods.

Farmers also warned that crops of wheat, barley, hay, potatoes, peas and soft fruits were threatened by heavy rains and waterlogged fields, with the damage done across Europe already pushing up prices for some cereals.

At lunchtime yesterday, there were five severe flood warnings in force along the river Don in South Yorkshire, and another 31 flood warnings. Environment agency spokesman Joe Giacomelli said the rain heading in from the west was now thought to be less severe than feared earlier, but that the agency remained on alert. 'It's better, but any further rainfall is going to exacerbate the situation.'

The agency said last week's flooding could not be blamed on poor defences alone because rainfall in some areas was at levels only expected once in every 150 years. In parts of Yorkshire, there was one sixth of the average June rainfall in 12 hours last week.

The Met Office said rainfall in June was two or three times the long-term average in parts of the country. The unsettled weather is expected to continue until the middle of this week.